Catecholamine-induced adipocyte lipolysis in human hyperthyroidism

Increased lipid mobilization in thyrotoxicosis is attributed to amplification of catecholamine action in fat cells by thyroid hormones. We investigated the adrenergic regulation of lipolysis in isolated sc abdominal fat cells obtained from 14 patients with thyrotoxicosis and 18 control subjects. Ten of the hyperthyroid subjects were also reinvestigated after antithyroid treatment. The thyrotoxic state was associated with a 3-fold increase in maximum norepinephrine-induced lipolysis (P < 0.005), unaltered sensitivity to dobutamine (selective beta 1-adrenoceptor agonist) and clonidine (selective alpha 2-adrenoceptor agonist), but 15 times enhanced sensitivity to terbutaline (selective beta 2-adrenoceptor agonist; P < 0.01). Moreover, thyrotoxicosis was accompanied by a 3-fold increase in beta 2-adrenoceptor number (P < 0.005), but unchanged beta 1-adrenoceptor levels. Further, the lipolytic effects of dibutyryl cAMP (activating protein kinase A and thereby hormone-sensitive lipase) and forskolin (activating adenylate cyclase) were about 60% enhanced (P < 0.005). No change in the maximum activity of the hormone-sensitive lipase could be demonstrated in the hyperthyroid state compared to that in the euthyroid state. The observed abnormalities in lipolysis and beta 2-adrenoceptor number were normalized after antithyroid treatment. It is concluded that in human hyperthyroidism, the interactions between thyroid hormone and catecholamines in adipocytes involve abnormalities at both receptor and postreceptor levels. The former mechanism seems to be a selective increase in the expression of the beta 2-adrenoceptors. The latter mechanism involves increased ability of cAMP to activate hormone-sensitive lipase, but not a change in maximum enzyme capacity.